This invention relates to an image magnification control device for a camera, wherein a photographic lens for a camera, including single lens reflex cameras and video still cameras, is driven and controlled by a zoom drive means so as to automatically control an image magnification of the photographic lens to a predetermined magnification.
A desirable feature for a power zoom camera is to be able to maintain a constant image magnification of an object as the distance between the object and the camera varies. For instance, when taking a series of continuous photographs, of for instance, a baseball player running to catch a ball, the photographer may wish that the baseball player occupies the same area of the film frame as the player moves. This can be accomplished by varying the focal length of the lens as the player moves. Unfortunately, an individual cannot change the magnification value (by changing the focal length of the lens) fast enough and accurately enough to achieve the desired result.
Over the years, camera manufacturers have developed power zoom lenses for lens shutter cameras. This enables a photographer to quickly and smoothly change the focal length of the camera lens. Lens shutter cameras typically employ a triangulation technique to determine the distance of an object to be photographed from the camera. In the triangulation technique, a source of light, such as infrared or ultrasonic, is emitted by the camera. The light source is bounced off of an object to be photographed and returned to the camera. By utilizing the triangulation technique, the distance "a" of the object can be determined.
The magnification value of a lens shutter camera is determined by an equation m=f/a, where "m" equals the magnification value, "f" equals the focal length of the lens and "a" equals the distance of the object from the camera. If a constant picture image size is to be maintained, the lens shutter camera must merely determine the distance of the object from the camera and change the focal length of the lens according to the above equation, so that the magnification value, "m", remains constant. This is easily accomplished with a power zoom, auto-focus, lens shutter camera.
Many individuals prefer using interchangeable lens cameras, such as SLR cameras, because a plurality of different lenses can be easily attached to a camera body. It would be desirable to enable an interchangeable lens camera to also be able to maintain the constant picture image size as the object to be photographed moves relative to the camera. However, constant image magnification techniques that are applicable to lens shutter cameras are not applicable to interchangeable lens cameras.
In an interchangeable lens camera, triangulation techniques are generally not used for focusing purposes because of inherent inaccuracies in such a system. Such inaccuracies are acceptable when a small size lens (i.e., 70 mm) is used, but is unacceptable when a large size lens (i.e., 200 mm) is employed. Due to the intrinsic errors of the triangulation technique, manufacturers developed a technique for focusing an object based on the amount of defocus of the object to be photographed. Two such defocus techniques are a phase difference detection (PDD) method and a contrast difference (CD) method.
A camera employing the defocus technique for focusing does not determine the distance "a" of an object from the camera. Thus, it is not possible to maintain a constant image magnification value "m" as the distance "a" of the object changes, using the techniques developed for lens shutter cameras (i.e., m=f/a). Because it is desired that the typical interchangeable lens camera be able to maintain a constant image magnification value as the distance of an object to be photographed from the camera moves, an alternative system must be developed.
Two methods exist for making an image on a film frame occupy the same amount of space, even though the distance between the object to be photographed and the camera varies. In the first method, the photographer preliminarily sets the size of the image before taking a picture. For instance, the photographer can select a portrait mode of operation, after which all pictures that are taken will be of the portrait type. In the second method, the camera photographer views the size of the image (as shown in the viewfinder) and adjusts the focal length of the lens until he likes the size. Then, the operator "fixes" the size. Thereafter, even if the object moves or the lens zooms, the image size of the object will be fixed. The present invention embodies both methods for making the image constant.
An image magnification control device for a camera, using a linking mechanism for a zoom lens device is disclosed, for example, in Japanese Patent Publication No. SHO60-1602. In this prior art, a cam mechanism provided in a zoom lens controls the amount of zooming of the zoom lens so as to keep the ratio of a real subject length and a real focal length constant. Moreover, in this cam mechanism, a cam surface provided in a lens barrel is formed in a shape of a logarithmic curve. By touching a linked roller which travels in the forward and backward directions in parallel with the lens axis to the cam surface using a spring, as the lens barrel rotates, the linked roller follows the cam surface. In addition, a variable resistor is linked with the linked roller so as to control the amount of zooming by means of such a variable resistor.
However, it is preferred that the position where the linked roller touches the cam surface be at the intersection of a plane including the center line of the linked roller and the lens axis and of the cam surface.
However, there is a limit to how much one can decrease the radius of the linked roller. Since the cam surface is in the shape of a logarithmic curve, rather than a linear shape, the position where the linked roller touches the cam surface deviates from the intersection described above due to a change of slope of the cam surface. Thus, there is a tendency for the amount of deviation to depend on the degree of the slope of the cam surface. Such a deviation is undesirable for a precise zooming control.
In addition, it is very difficult to accurately machine a cam surface, with a high precision, in the shape of a logarithmic curve.
Further, a camera which is equipped with a CPU used for conducting an autofocus control and process control has been developed. For such a camera, motors therein may drive a zoom lens and conduct the image magnification.
When a release process is performed while a subject is out of the range where the image magnification can be controlled, a photograph is taken in the condition where a predetermined image magnification is not obtained, resulting in an undesirable situation.
In this case, when a subject moves out of a range where the image magnification can be controlled, the image magnification control is temporarily stopped. When the subject moves back too many spaces into the allowable range of the image magnification control, it is desirable to resume the image magnification control. In addition, it is preferred that this operation be conducted simply.
Moreover, there has been a camera which is equipped with an interchangeable lens. For such a camera, motors therein may drive a zoom lens and conduct the image magnification.
In the meantime, when such an image magnification control is performed in a sequence shot mode, if a subject moves after it has been focused and until the next release process is conducted, the next photograph would be unfocused.
Further, new types of electronically controlled cameras have been developed having features, such as autofocus control and program control. Conventionally, a camera has been known wherein a focus lock takes place when a subject is focused and a light metering operation is started by turning ON a light metering switch in an auto and in-focus priority mode. As another prior art camera, a camera has been devised wherein pressing an image magnification mode setting switch allows an autofocus operation and power zoom operation to take place regardless of the distance to a subject so as to automatically control an image magnification of a photographic lens. By combining the functions of the above cameras, the performance of such cameras can be enhanced.